Ball bearings

ABSTRACT

An angular contact ball thrust bearing has its outer race formed in two parts each of which has an arcuate track, the centers of the arcs comprised in the tracks being offset, the profile of the race with which the balls make contact thus not constituting a continuous circular arc. In a substantially unloaded condition each ball makes peripheral contact with the outer race at a point on one track substantially diametrically opposite to a contact point on the inner race. Under operational loading conditions, resultant forces on the balls urge them into peripheral contact with a second point on the outer race on the other track. The two arcs may be of differing radii while the outer race may be in either one or two parts and may include a substantially continuous elliptic arc in place of the two circular arcs.

United States Patent Haines Mar. 7,1972

[54] BALL BEARINGS [21] Appl. No.: 30,892

1,356,444 10/1920 Golden .:::308/193 BEARING AXIS.

FOREIGN PATENTS OR APPLICATIONS 977,603 11/1950 France ..308/l93 PrimaryExaminer-Edgar W. Geoghegan Assistant Examiner-Frank SuskoAttorney-Cameron, Kerkam & Sutton [57] ABSTRACT An angular contact ballthrust hearing has its outer race formed in two parts each of which hasan arcuate track, the centers of the arcs comprised in the tracks beingoffset, the profile of the race with which the balls make contact thusnot constituting a continuous circular arc. In a substantially unloadedcondition each ball makes peripheral contact with the outer race at apoint on one track substantially diametrically opposite to a contactpoint on the inner race. Under operational loading conditions, resultantforces on the balls urge them into peripheral contact with a secondpoint on the outer race on the other track. The two arcs may be ofdiffering radii while the outer race may be in either one or two partsand may include a substantially continuous elliptic arc in place of thetwo circular arcs.

7 Claims, 11 Drawing Figures PATENTEDMAR 71.972 3,647,268

SHEET 3 nr 6 PATENTEDMAR 7 I972 SHEET [1F 6 FIG. 4b.

PATENTEDMAR 7 I972 SHEET 5 BF 6 23 i i i BALL BEARINGS This inventionrelates to ball bearings in which axial thrust loads are transferredthrough the balls from one race to another, and is particularlyconcerned with angular contact bearings in which a line joining thecontact points of a ball and the tracks of the races (usually adiameter) is inclined to a transverse plane normal to the bearing axis.

In gas turbine aeroengines where high-rotational speeds under load areinvolved, it is particularly the case that bearing life has become acritical factor in further development.

There has been a lack of fundamental knowledge as to the precise motionof balls in races, especially in angular contact thrust bearings. It ispossible to calculate the position which balls should occupy in a raceby making fairly sweeping assumptions but lack of reliable data on theactual positions under operating conditions has made it extremelydifficult to apply such predictions in order to make realistic estimatesof ball motion and the angles subtended by the points of contactsbetween balls and their races have never been accurately known in thepast.

Bearings cease to function correctly when flakes of material becomedetached from the rolling surfaces, or when the tracks of the races andthe balls suffer surface damage.

It has been clear that in many cases balls have been sliding in theraces rather than rolling freely. This leads to breakdown of lubricationbecause sliding balls wipe oil away from the contact area whereupon theprotective oil film becomes incomplete. Surface asperities are bruisedand even become detached thereby changing ball diameter and the tracksurface condition; balls which have worn to an ellipsoidal shape inwhich the tracking portion is of smaller diameter are capable ofhammering a bearing cage to destruction. The friction accompanying slidecan also produce heat and other damaging effects.

It has been widely thought that slide under load primarily occurred atthe inner race of an angular contact thrust bearing with little or noneat the outer race. It has now been found, as a result of extensiveobservations of the motion of balls in an actual bearing, that extremeslide occurs at the outer race and is, moreover, a major cause oflimited bearing life; the problem is accentuated by the heavy loading atthe outer contact point arising from centrifugal forces. Oil films arefrequently punctured by asperities on the ball and track surfaces. Whenthis happens the rate at which surface damage occurs is extremelysensitive to contact loading.

The present invention is directed to decreasing outer race contactloadings under operating conditions by the addition of a further contactpoint whereby the normal outer race contact point is relieved of aconsiderable proportion of the loading induced by centrifugal forcesacting on the balls at high-rotational speeds.

An angular contact ball thrust bearing according to the inventionincludes a row of balls and an outer race shaped to make contact witheach ball at two separate peripheral positions when the bearing issupporting the axial loads for which it is designed and normallyoperates.

In a preferred embodiment, the profile of the outer race comprises twoeccentric circular arcs each in contact with the balls under design loadconditions.

The circular arcs may be of the same, or differing radii while the outerrace may be constructed in two separate parts.

In another embodiment, the profile of the outer race includes asubstantially continuous elliptic arc.

In order that the invention may be more clearly understood, referencewill now be made to the accompanying diagrammatic drawings, of which:

FIG. la illustrates contact conditions between a ball and a track of arace under simple rolling,

FIG. lb is a view of part of the race of FIG. In looking in thedirection of the arrow D therein and showing the contact area,

FIGS. 2a and 2b are views corresponding to those of FIGS. la and lbrespectively under conditions of simple rolling combined with simplespin,

FIG. 3 is a view corresponding to FIGS. 1b and 212 under gyroscopiceffects,

FIG. 4a is an axial section through part of an angular contact ballbearing,

FIG. 4b is a view of a ball and part of the inner race of FIG. 4alooking in the direction of the arrow D therein (corresponds to FIGS. lband 2b).

FIGS. 5 and 6 are similar transverse sections through part of aconventional angular contact ball thrust bearing showing the effect ofdiffering operating conditions, and

FIGS. 7 and 8 are views corresponding to FIGS. 5 and 6 of an angularcontact ball thrust bearing incorporating the invention.

FIG. 1a shows on a large scale a ball 1 in contact with an arcuate track2 extending round an annular race 3. FIG. lb shows a contact surface 4which is formed between the ball 1 (not shown) and the track 2. Thecontact surface 4 has a finite area due to elastic deformation of theball and track and is elliptical in shape. Under simple rollingconditions the ball I rotates about an axis 5 coplanar with the axis ofthe bearing of which the ball and race 3 form part (the dotted line BBin FIG. lb indicates the common plane of the rolling axis 5 and thebearing axis), and behaves on the track virtually as if rolling along astraight groove. Because of the curvature of the track profile, however,the contact surface 4 is not flat and the ball will thus not roll freelyat all points within the contact surface. With the track moving relativeto the ball in the direction of the arrow B in FIG. lb, the ball willslide' backwards at the center of the contact surface as indicated bythe arrow S and slide forwards at the radial (relative to the bearingaxis) extremities of the contact surface as indicated by the arrows P,the said slide directions being separated by two lines of nominal purerolling x and y.

In simple rolling a nominal tangent 6 (FIG. la) to the ball intersectsthe rolling axis 5 on the axis of the bearing. The nominal tangent 6subtends an angle 11, with the rolling axis 5 and cuts the contactsurface 4 at points X and Y (FIGS. la and lb) on the lines x and y (FIG.lb) respectively. If free rolling of the ball 1 is disturbed, forinstance by friction between the ball and a cage in which it is carried,then the lines of nominal pure rolling x, y'(and consequently the pointsX, Y) will move closer together or farther apart depending on whetherthe track is a driving or driven (or reacting) member in the bearingassembly.

Spin is a condition when a ball turns or tends to turn about an axisother than its simple rolling axis while rotating between tracks in abearing.

When simple spin is superimposed on simple rolling there will bedisplacement of the rolling axis. FIGS. 2a and 2b (in which the samereference letters and numerals as in FIGS. Ia and 1b indicatecorresponding features) the ball 1 is rotating in a counter clockwisedirection relative to the track 2 according to the view of FIG. 2b, asindicated by the arrow H. Rolling now takes place about an axis 7 whichhas been displaced relative to the previous rolling axis 5 by an angle0,, but remains in the same plane (BB) as before. In FIG. 2b the contactsurface 4 is shown as being in the same location as previously thoughtin practice there might be some slight movement. Displacement of theaxis results in nominal pure rolling only occurring along the line x,the line y (not shown) now being outside the contact area. A nominaltangent 8 intersecting the rolling axis 7 on the axis of the bearing andcuts the line x at point X (which would not normally be coincident withits position in FIGS. la and lb).

The position of line x and the intercept X is again dependent on effectssuch as friction between ball and cage and whether or not the track is adriving or driven member. In this case the ball 1 is effectivelyspinning about the point X and there will be a resultant tendencytowards damage to ball and track.

In complex spinning conditions it is likely for there to be no line ofnominal pure rolling within the contact surface 4, in which case ballspin will be associated with large tangential sliding velocities betweenball surface and track.

Gyroscopic effects generated in balls rotating at high speed around theaxis of a bearing can cause inclination of the rolling axis of a ballout of the plane of the bearing axis. Such a condition is indicated inFIG. 3, a rolling axis denoted by the line AA being inclined to bearingaxis plane BB by the angle [3. The result is a bodily movement of theball in question across the track 2 in a sideways direction relative totrack movement (transverse slide) as indicated by the arrow K. Nominalsimple rolling is not possible and puncturing of an oil film betweenball and track will usually result in severe surface damage. The rate ofdamage is conditioned according to whether or not oil is subsequentlycentrifuged" off the surface of the track (in the "direction of thearrows W) by track rotation, and if sideways movement of the ball thenassists or retards the centrifuging action.

For convenience only a bearing inner race and track has been shown inthe foregoing Figures but comparable conditions exist between a ball andthe outer track of a bearing. FIGS. 40 and 4b show a conventionalangular contact ball bearing setting out the principal parametersaffecting ball behavior in relation to the tangents at ball to trackcontact surfaces. The same conventions, reference letters and numeralsare used as previously in relation to corresponding features. Inaddition, an outer race 9 having an arcuate track 10 results in arolling axis 11 which corresponds to the rolling axis shown in relationto the inner race 3 in FIG. 1a, and a nominal tangent 12 subtending anangle 11,, to the rolling axis 11 in similar fashion.

The notation used in FIGS. 4a and 4b which is consistent with that inprevious Figures where appropriate, is as follows:

AA typical ball rolling axis at high-ball rotational speeds BB ballrolling axis coplanar with bearing axis GG ball translation axis a, aangles defining ball spin at inner and outer track contact surfaces (1TozT inclination of tracking bands to the bearing axis (=90contact angle)B inclination of ball rolling axis between AA and BBarising fromgyroscopic effects and causing ball slide across the bearing tracks(i.e., transverse slide) v v angles defining simple rolling at inner andouter track contact surfaces inclination of ball-rolling axes to thebearing axis 0,0 displacement of ball-rolling axes due to spin 1" ri u'0' 'o) w w components of ball angular speed about S Spin AngularVelocity pm m Ball Rolling Angular Velocity tan 6, at inner track andtan 6 at outer track Velocity of slide Transverse slide ratio (BallRolling Velocity) war. t l B at inner trac (1)33 COS do Sll'l GTO) k tanB d at outer track The only complete theoretical analyses of ballbehavior have hitherto assumed that the motion of a ball is controlledby a state of simple rolling at the outer contact surface (outer racecontrol). with B=0 =0, a state of simple rolling at the inner'contactsurface (inner race control) with B=0,=0, or a state of control in whichB==0 and both 0, and 6 are positive angular quantities (sometimes called"shared race control").

It can now be shown that these assumptions, which form the basis ofcurrent bearing design, are invalid under high-speed race usageconditions because the gyroscopic couple acting on a ball can generate aB value which causes m and a slow rotation of the ball in the centerlineplane of the bearing with the result that cage speed, which customarilyhas been used in postulations as to the nature of the control existing,appears to approach that for an inner race control condition. However,the value of 6, falls and probably changes sign so that the concept ofball control at one or other of the two contact points, or the conceptof a stable condition with both 0, and 0 0 is I not valid. Experimentsshow quite clearly that the condition is associated with a definitetransverse sliding of the ball at the inner race contact points, to allintents and purposes independent of thrust loads over a wide range.

In FIGS. 5 and 6, an angular contact ball thrust bearing comprises incoaxial relationship an inner race 21, a row of balls (of which one isshown at 22) and an outer race 23. The races 21, 23 have arcuate tracks24 and-25 which respectively make contact with the balls at oppositepints on the surfaces thereof; each of the tracks 24, 25 is constitutedin profile by a continuous circular arc.

FIG. 5 shows a typical loading where the balls are stationary around theaxis of the bearing. Thrust forces, indicated by the arrows T, acting onthe races in an axial direction are transmitted through the balls, viacontact surfaces indicated by the arrows C, and C along a ball diameterwhich is inclined relative to the axis of the bearing. The loadings atthe contact surfaces are balanced and tangents from the contactsurfaces, indicated by the chain dotted lines 26, 27, and which indicatethe positions of the respective rolling axes, are parallel with eachother. This means that the balls will roll with shared control at boththe inner and outer contact surfaces with one or both races rotating atslow speed about the bearing axis.

At high-rotational speeds (such as are likely to be experienced in gasturbine aeroengines under normal operating conditions), high-centrifugalforces will be generated in the balls. These forces act radiallyoutwards relative to the bearing axis and cause the balls to sit deeperin the outer race with the result that thecontact surface a is displacedas shown in FIG. 6. There will be some displacement of the contactsurface C on the inner race also.

Thrust forces together with radial loading (as indicated by the arrow R)and gyroscopic effects generated by the rotation of the balls about theaxis of the bearing are transmitted through the balls to produce anunbalanced force and couple system giving both ball spin andsimultaneous transverse ball slide.

In a bearing having steel balls at a pitch circle diameter of 6.484 in.,a stationary outer race, an inner race speed of 20,000 rev./min, anominal ball diameter of 0.750 in., and a conformity ratio of balls totracks of 97.3 percent, observed experimental values with a thrust loadT of 250 lb, per ball were; centrifugal force, indicated by arrow F, of563 lb, per ball, an inner track loading at contact C, of 431 lb and anouter track loading at contact C of 947 lb,. The tangents 26, 27 are notnow parallel and consequently extreme ball spin (with resultant ball andtrack damage) and, due to inclination of the ball-rolling axes out ofthe plane of the bearing axis, extreme ball slide will occur at theheavily loaded outer track (affecting both ball and track).

In the bearing shown in FIGS. 7 and 8, in which the same referencenumerals are used as in FIGS. 5 and 6 to indicate correspondingintegers, the inner race 21 and balls 22 are substantially the same asin FIGS. 5 and 6. The outer race, however is in two parts 28, 29 whichare disposed symmetrically about a transverse plane extending radiallyfrom the bearing axis. Each of the parts 28, 29 has an arcuate track,30, 31 respectively, whose radii, r r;,, are greater than that, r of theballs and are struck from noncoincident centers L,M, the tracks thus notconstituting a continuous circular arc.

In FIG. 7, where the operating conditions correspond to those of FIG. 5,thrust is transmitted between the inner race 21 and the part 28 of theouter race through contact surfaces C and C, in similar fashion, andsimple rolling conditions likewise exist at the contact surfaces so thatthere is almost no surface rubbing (and wear) at these points. Normallyunder these conditions there will be no contact between the balls andthe track 31. Any ball spin occurring in the event of such contactshould, however, be at a nondamaging load level.

At high-rotational speeds, such as are experienced in gas turbineaeroengines, centrifugal forces generated in the balls will result inthem sitting deeper in the outer race as before when contact will bemade with track 31 in the part 29 at C,, as shown in FIG. 8. The effectof this is to counteract the unbalance tending to displace the rollingaxis and maintain the contact surface C in approximately the samelocation on its track throughout. Within the design operating range, thecontact surfaces C,, C, will be approximately midway along therespective tracks, the resultant loading giving a substantially balancedforce and couple system with the net result that the contact surfaces CC are displaced only slightly from the positions they occupy in the slowspeed case (FIG. 7) and thrust forces continue to be transmittedsubstantially diametrically through the balls with the tangents 26,27 tothe contact surfaces parallel to each other, or even slightlyconvergent. There will be ball spin at all three contact surfaces butlittle or no slide will occur between the inner race 21, the part 28 andthe balls.

In an observed test situation where dimensions and operating conditionscorresponded with those described in relation to FIG. 6, contact loadsrecorded were respectively C 437 1b,, C,408 lb, and C -722 lb, at 20,000rev./min.

A comparison of energy generation for the above example and that of FIG.6 shows that the total energy generation by friction is reduced toapproximately half where three-point contact occurs in the operatingcondition, a major reason for the decrease being the reduction inloading at contact surface C, and a consequential increase in theefficiency of lubrication.

Conformity ratios of balls to tracks (conformity ratio=ball radius/trackradius) are normally between 85 and 99.5 percent. I-Iigh-conformityratios reduce contact stresses but (in the presence of spin at a contactsurface) can increase the possibility of damage to balls and trackbecause of the larger contact areas involved and some compromise valueis usually adopted.

As has already been indicated, the thrust carrying contact surfaces C C,do not significantly change position with variation of operatingcondition. For example, with balls of the order of l in. diameter andconformity ratios of the order of 90 to 95 percent, the angles subtendedby the said contact ball points to ball diameters extending radiallyfrom the bearing axis would not be expected to change by more than 10 atinner race speeds from to 20,000 rev./min with thrust loads of up to1,000 lb. per ball.

This compares with a displacement in the angle of an outer race contactsurface (C of 20 percent or more in conventional bearings such as inFIGS. and 6, with more than percent variation in the angle of an innerrace contact surface 1)- The angle subtended by the third contactsurface (C however may change under load.

In general, the radii of the tracks 30, 31 (r r shown in FIGS. 7 and 8will be the same but it can be envisaged that these could differ. One orboth tracks can contain more than one arc form and the distance from abearing centerline to the center L (FIG. 7) may differ from the distancebetween the bearing centerline and the center M.

The outer race shown in FIGS. 7 and 8 is formed in two parts but this isprimarily for ease of manufacture and is not essential, the race can bein one piece provided that those portions of the track at which the twocontact surfaces C C occur is not a continuous circular are. In oneembodiment having a one-piece outer race there are two tracks defined bytwo eccentric circular arcs substantially as described in relation tothe split outer race of FIGS. 7 and 8 while a continuous elliptic arc isused in another embodiment.

The present invention goes counter to established teaching in bearingdesign in requiring ball contact at two points on the outer race of anangular contact thrust bearing in operating conditions. What are termedthree-point and four-point contact bearings are well-known but thesenames are only true in the static case or when under purely radialloading. It has always been specified that there will be only one pointof contact at the inner and outer races when thrust loads are applied.

The use of split races for ease of assembly is fairly common practiceand advantage is frequently taken of this in removing a small amount ofmaterial from the adjacent faces to effect a reduction in end float suchas occurs with reversal of thrust in a bearing required to support axialthrust loads in two directions. The loadings applied have hitherto notbeen of such magnitude as to give other than two-point contact betweenballs and races under design operating conditions. Should three-pointcontact occur, this would only be because of excessive loading such aswould be likely to cause failure, or malfunction of the bearing. Infact, indications are that operation with a significantly loaded thirdcontact surface at relatively low-shaft rotational speeds is at leastunwise.

What I claim is:

1. An angular contact ball thrust bearing comprising a row of ballsdisposed between inner and outer races, in which the outer racecomprises two ball-contacting tracks of arcuate form so arranged thatunder axial loading alone each ball is in peripheral contact at only twosubstantially diametrically opposite points with the inner race and onetrack of the outer race respectively, a line joining said contact pointsbeing inclined to a transverse plane normal to the bearing axis, and theouter race is so shaped that an increase in centrifugal force on theballs causing displacement of the aforesaid contact points acts to bringeach ball into peripheral contact at a third point with the second trackof the outer race at a predetermined loading corresponding to theoperational rotational velocity of the bearing, forces generated by ballcontact with the second track acting to counteract displacement of ballcontact points with the first-mentioned track.

2. An angular contact ball thrust bearing according to claim 1 in whichthe ball contact points on the two tracks of the outer race are disposedon opposite sides of a transverse plane extending through the centerlines of the balls.

3. An angular contact ball thrust bearing according to claim 1 in whichforces generated in the outer race due to ball peripheral contact withthe second track act in opposition to gyroscopic effects resulting fromball rotation about the bearing axis and tending to displace a ballrolling axis from the instantaneous axial plane extending through thebearing axis and the center of the ball.

4. An angular contact ball thrust bearing according to claim 1 in whichthe profile of each track is a circular arc and the arcs of the twotracks are eccentric.

5. An angular contact ball thrust bearing according to claim 1 in whichthe profile of the outer race is a substantially continuous ellipticarc.

6. An angular contact ball thrust bearing according to claim 1 in whichthe outer race is constructed in two separate parts, one part being incontact with the periphery of each ball at all material times.

7. An angular contact ball thrust bearing according to claim 1 in whichthe outer race is constructed in two parts and one track is formed oneach part.

1' 1! i it

1. An angular contact ball thrust bearing comprising a row of ballsdisposed between inner and outer races, in which the outer racecomprises two ball-contacting tracks of arcuate form so arranged thatunder axial loading alone each ball is in peripheral contact at only twosubstantially diametrically opposite points with the inner race and onetrack of the outer race respectively, a line joining said contact pointsbeing inclined to a transverse plane normal to the bearing axis, and theouter race is so shaped that an increase in centrifugal force on theballs causing displacement of the aforesaid contact points acts to bringeach ball into peripheral contact at a third point with the second trackof the outer race at a predetermined loading corresponding to theoperational rotational velocity of the bearing, forces generated by ballcontact with the second track acting to counteract displacement of ballcontact points with the first-mentioned track.
 2. An angular contactball thrust bearing according to claim 1 in which the ball contactpoints on the two tracks of the outer race are disposed on oppositesides of a transverse plane extending through the center lines of theballs.
 3. An angular contact ball thrust bearing according to claim 1 inwhich forces generated in the outer race due to ball peripheral contactwith the second track act in opposition to gyroscopic effects resultingfrom ball rotation about the bearing axis and tending to displace a ballrolling axis from the instantaneous axial plane extending through thebearing axis and the center of the ball.
 4. An angular contact ballthrust bearing according to claim 1 in which the profile of each trackis a circular arc and the arcs of the two tracks are eccentric.
 5. Anangular contact ball thrust bearing according to claim 1 in which theprofile of the outer race is a substantially continuous elliptic arc. 6.An angular contact ball thrust bearing according to claim 1 in which theouter race is constructed in two separate parts, one part being incontact with the periphery of each ball at all material times.
 7. Anangular contact ball thrust bearing according to claim 1 in which theouter race is constructed in two parts and one track is formed on eachpart.